Chevrel phases, structure

Lower formal oxidation states are stabilized, however, by M-M bonding in ternary chalcogenides such as M MeQn, M4M6Q13 (M = alkali metal M = Re, Tc Q = S, Se) and the recently reported M gMeS. Their structures are all based on the face-capped, octahedral MeXg cluster unit found in Chevrel phases (p. 1018) and in the dihalides of Mo and W... 

Fig. 6.1 The fundamental structural unit found in the Chevrel phases (cluster MoeXg full circles Mo atoms) displayed in three ways to emphasize different views of the connectivity. In (a) an octahedron of molybdenums (Mo-Mo = 2.7 A) is encased in a cube of chalcogens (Mo-S 2.45 or Mo-Se 2.6 A). Scheme (b) exhibits the same cluster as consisting of an octahedron with its triangular faces capped by chalcogenides. In (c), the cluster has been reoriented so that a threefold axis is vertical. (Reproduced from [10])...

The different classes of Ru-based catalysts, including crystalline Chevrel-phase chalcogenides, nanostructured Ru, and Ru-Se clusters, and also Ru-N chelate compounds (RuNj), have been reviewed recently by Lee and Popov [29] in terms of the activity and selectivity toward the four-electron oxygen reduction to water. The conclusion was drawn that selenium is a critical element controlling the catalytic properties of Ru clusters as it directly modifies the electronic structure of the catalytic reaction center and increases the resistance to electrochemical oxidation of interfacial Ru atoms in acidic environments. 

Most of the chalcogenide studies have involved binary, ternary, and pseudo ternary metal clusters. The ternary Chevrel-phases can be represented by MxMo6X8 (M = transition metal, X = chalcogen atom), where the transition metal fills the cavities in the Mo6X8 crystal structure. The pseudo-ternary (or pseudo-binary) Chevrel-phase clusters can be represented by MxMo6 xX8, where the Mo atoms are substituted by transition metal atoms.185 The crystal structure of ternary and pseudo ternary Chevrel phases are shown in Fig. 4. The carbonyl-chalcogenide clusters189 192 will be discussed in the next section. 

Fig. 4 The crystal structure of Chevrel phase chalcogenides. The atoms are represented by (Mo), O (chalcogen) and A (site of the metal atoms for the ternary Chevrel phases). For pseudo-binary phases, the Mo is partially substituted by the metal atoms. (181 With kind permission of Springer Science and Business Media).

Trends in the electronic structure of the chalcogenide catalysts have proved to be helpful in the design and understanding of the catalyst clusters. During ORR, the molecular oxygen has been found to react with the cluster as a whole, rather than individual metal atoms.177 The overall number of electrons per cluster unit (NEC) in the valence bond has been shown to have a factor in the activity and stability of the cluster catalysts.177,181 The unsubstituted Chevrel phases have a NEC of 20.177,181 Substituting or intercalating other transition metals into the crystal lattice to make ternary or pseudo-binary Chevrel phases allows for the increase of NEC. It has been found that as the NEC approaches 24, the catalytic activity improves.181 Alonso-Vante compiled the results from his previous studies to show the effect of NEC in... 

The nearly cubic, rhombohedral unit cell of Mo6PbS8 is shown in Fig. 4.30. The bonding between different clusters in the crystal structure of the Chevrel phase Mo6PbS8 is indicated in Fig. 4.31. 

Cu4Mo6Seg. The triclinic structure of these compounds has been described by McGuire et al. (2006). It contains nearly regular Mo6 clusters the Cu atoms fully occupy four sites between faces of two adjacent Mo6Se8 clusters. This compound is a member of a series of MexMogX8 Chevrel phases. Preparation and properties have been discussed. A Cu intercalation apparatus was built for... 

Figure 4.31. Crystal structure of the Chevrel phase Mo6PbS8. Portions of four rhombohedral unit cells with one common Pb atom (black) are shown. The bonding between different clusters is suggested by the Mo—S inter-cluster links.

The Chevrel phases are structure containing clusters as building blocks and they may be considered as phases having characteristics at the borderline between those of compounds containing molecular-like units and of solids containing infini-tively extended units. Such phases have been described in 4.4.4.7 together with Mo6X8 clusters and the products of their condensation. 

The structure of the copper Chevrel phase is shown in Fig. 7.7. It is based on Mo Sg building blocks which stack in three dimensions. A... 

X-ray crystallography, 40 20-21 synthetic models, 40 23-48 xanthane oxidase, 40 21-23 chalcogenide halides, 23 370-377, 413 Chevrel phases, 23 376-377 metal-metal bonding, 23 330, 373 structural data, 23 373-376 as superconductors, 23 376 synthesis, 23 371-372 chloride, 46 4-24, 35-44 heterocations of, 9 290, 291 cluster compounds, 44 45-46 octahedral, 44 47-49, 53-63 electronic structure, 44 55-63 molecular structure, 44 53-54 synthesis, 44 47-49 rhomboidal, 44 75-82 solid-state clusters and, 44 66-72, 74-75, 80-82, 85-87 tetrahedral, 44 72-75 triangular, 44 82-87 cofactor, 40 2, 4-12 anaerobic isolation, 40 5 molybdopterin and, 40 4-8 reduced form, 40 12 synthesis, 40 8-12 xanthine oxidase, 45 60-63 complexes... 

In passing, it should be noted that Michel and Raveau had carried out electrical measurements on several alkaline-earth substituted La2Cu04 compounds and presented (140) their experimental results in a comprehensive review during 1984. Their work, however, was primarily structural with a focus on electrical and magnetic properties above 50 K for this reason they missed observing the superconducting transition at 30 K. It is this author s belief that these Cu-O phases should be called the "Raveau-Michel" phases in much the same way as the molybdenum sulfide superconductors are called the "Chevrel" phases. 

Figure 1.42 Structure of M MOgXg Chevrel phases (a) the MOgXg building unit and (b) the stacking of eight MogXg units in the rhombohedral structure. The cube-shaped cavity at the centre is occupied by the large M atom. (After Yvon, 1979.)...

There are several interesting families of inorganic mixed-valence compounds that we have not discussed here (see Yvon, 1979 McCarley, 1982). For example, there are metal-cluster compounds such as the Chevrel phases, M,jMo6X8(X = S or Se) and condensed metal-cluster chain compounds such as TlMojScj, TijTe, NaMo O and M PtjO. TTF halides and TTF-TCNQ complexes (Section 1.9) constitute molecular mixed-valent systems in which the mixed valency is associated with an entire molecule the charge on TTF in such compounds is nonintegral. The structure of TTF-Br(, 79 and... 

---